The Absolute Spectral Diffuse Reflectance of Magnesium Oxide*

Abstract

The published values of the spectral diffuse reflectance of magnesium oxide indicate that it should reflect more highly in the red than in the blue. Using a modification of the method described by Preston, we have measured the absolute value of this quantity between λ=0.24μ and λ=0.85μ. A very carefully prepared surface of MgO on silver has a reflectance of about 0.98 at λ=0.54μ, where there is a very flat maximum. The reflectance of a fresh surface falls to less than 0.96 at 0.4μ, but exposure to strong ultraviolet light bleaches it, the final result being a reflectance increasing continuously from the near-infrared to about 0.4μ, where it begins to fall sharply, reaching 0.96 at about 0.25μ. A sphere lined with such a bleached surface raises the color-temperature of Planckian radiation a little, and the observed color agrees well with that predicted from the new results. It is believed that these results are reliable to ±0.002 from λ=0.4μ to λ=0.7μ, but different surfaces prepared with equal care may differ in reflectance by at least 0.005.